Bonding is the surface-to-surface joining of similar or dissimilar materials using a substance which usually is of a different type, and which adheres to the surfaces of the two adherents to be joined [1]. According to DIN EN 923, an adhesive is a non-metallic substance capable of joining materials by surface bonding (adhesion), and the bond possessing adequate internal strength (cohesion) [7].
The expression adhesive may be used interchangeably with glue, cement, mucilage or paste and is any substance applied to one surface, or both surfaces, of two separate items that binds them together and resists their separation [18]. The basic definition of an adhesive as used by the Adhesive Sealant Council in America is
Figure 5 Types of Joints commonly used [1]
“A material used for bonding that exhibits flow at the time of application”. Adhesive bonding technology offer great design flexibility as it can be integrated into any complex design and in almost all available industrial sequence of mass production. Over the years as the advancement of knowledge in adhesive science, specific adhesives have been developed that bind very strongly both to organic and inorganic materials. Adhesive bonding can rarely compete with other joining techniques used in the industry.
For example, adhesive bonding applications are limited when it comes to bonding a steel bridge, whereas for lightweight construction of car bodies, joining of aluminium, glass and plastics, adhesive joining offers extremely interesting applications [1].
One of the major advantages of bonding is that little or no heat is necessary to create the joint [7]. When adhesive bonding systems are used for joining, the material (substrate) structure is not macroscopically affected. There are less risks of deflections or internal stresses – which are generally related to the application of heat on the substrates. The disadvantage which is notable when adhesive bonding is used is that the bond has a limited stability to heat and specific production requirements (parameters) have to be met to achieve a successful bond with high strengths.
3.1 Classification of adhesives
There are many methods of classifying adhesives. Some use a system of end use, example adhesives for metals, adhesives for textile, adhesives for plastics etc. However, a system based on the chemical properties, performances and curing mechanism is more appropriate and will be used in this thesis.
3.1.1 Non-curing (pressure-sensitive) adhesives
Pressure-sensitive adhesives belong to the group of highly viscous polymer systems. They partly retain the properties of liquid in their final state, which allows them to completely adapt to the
adhesive bonds [1]. They are usually designed to form a bond and hold properly at room temperatures, also they typically reduce or lose their bonding strength at low temperatures and reduce their shear holding ability at high temperatures [19].
3.1.2 Physically setting adhesives
If the transition from the liquid state of the adhesive to the solid state takes place by physical processes such as evaporation, solidification of the melt or diffusion processes without chemically changing the polymer components of the adhesive, it is termed as
‘physical setting’ [7].
Contact adhesives are a type of physically setting adhesives which are made from polymer components that are not yet chemically cross-linked. These adhesives should be applied to both surfaces of the adherents and solidification takes place by a drying process before the materials are joined. The adherents are pressed together as soon as the adhesive layers are dry to touch [18]. A diffusion process takes place as pressure is applied on the adherents, firmly joining the adherents together. The strength of bond is increase in hours after the application of pressure. If the surface of the substrate is modified (prepared), and there are irregularities on the surface, partial bond is formed even before the drying process has taken place.
Plastisol adhesives are a type of physically setting adhesives which are extensively used in the lightweight construction in the field of automobiles. These adhesives are formed by the dispersion of a polymer, mostly polyvinyl chloride (PVC), in a plasticizer.
During the curing process, when the dispersion is heated, the polymer dissolves irreversibly in the plasticizer that acts as a solvent, transforming into soft PVC. Additives can be added to improve the heat resistant characteristics of these plastisol adhesives. These adhesives are relatively cheap and high durability of the bond are obtained [7].
Hot-melt adhesives are a type of physically setting adhesives, which exist in a solid state and do not contain any solvents.
Application of these adhesives are done in the molten state in order to obtain an appropriate wetting of the adherent surface.
Immediately after cooling, hot-melt adhesives have a capability to transmit forces. They can be used at high heating and cooling rates to speed up the production in industries [1]. Hot-melt systems are widely used method to bond windscreens in automotive bodyworks. Hot-melts can be applied at low temperatures, while being able to resist temperatures of 120ᵒC - 150ᵒC in the cured strengthen by means of chemical reactions, which must take place after the application of the adhesive and after joining the adherents. The time needed for curing can be altered by the application of heat.
Two component adhesives are a type of chemically setting adhesives, which consists several components that are mixed in a specific ratio before application. The most important adhesives of this group are the polyesters, cold setting epoxy resins, polyurethanes and acrylic adhesives. For some adhesives, it is possible to apply each of the component to separate substrates being joined together [1]. The term ‘pot-life’ is important with two component adhesive systems, which gives an approximate period of time during which the adhesives can be used after mixing the components, this is in turn dependent on the type of adhesive and the volume prepared.
Cold setting, one component adhesives are a type of chemically setting adhesives, which employ a wide range of polyols. They are reacted with isocyanates to form an isocyanate - terminated pre-polymer. The resulting isocyanate – terminated pre-polymer is diluted in a non-hydroxyl containing solvent (such as ethyl acetate) and supplied to a converting operation in this form. The adhesive when applied must react with water (either from moisture from the air or from the substances used or by intentional moisture which is induced on the substrates) to form an amine-terminated intermediate that can further react with the isocyanate-terminated polymer. Typical curing rates for the one component adhesives range from 2-7 days [20]. These cold setting one component adhesives can also be initiated by irradiation or by an absence of oxygen. Under the influence of UV light, the system can be accurately controlled. Photo-initiators are dissolved or chemically incorporated with the adhesive systems, which initiate the cross-linking process under irradiation with UV or visible light [1]. The advantage of using cold setting one component adhesive is that it is virtually unaffected over a temperature range from -55ᵒC to +250ᵒC.
Hot setting, one component adhesives are a type of chemically setting adhesive, which consist of low molecular and plasticised substances [1]. This group of adhesives consists mainly of phenolic resins, epoxy resins and polyimides. The cross-linking process takes place in the form of poly-condensation, pressure and heat should be applied in order to remove the water from the bond line during the process. These adhesives are not capable of transmitting forces between the substrates before curing and thus any deformation of the substrate may result in destruction of the bond.